Can anyone tell me how to upload a PDF file with the post?

You can attach a file to either a new post or a reply; however files cannot be uploaded if you use the Quick Reply function at the bottom of a thread. You must click the Reply button. When you create a post, look for the File Manager link beneath the edit window, which allows you to attach your file.

Hmmmm... Can't find a File Manager link anywhere. I'm using Firefox Mozilla 5.0.

Perhaps this ver doesn't support it?

It's not a problem with your browser (I use Mozilla Firefox, too). I just checked the forum settings and found that attachments were not enabled for this forum when it was created. They are now.